Mine props which can withstand substantial bearing forces are widely used in mining operations and can be braced between the floor and a ceiling structure to be supported by a screwthread manipulation to extend the mine prop. Such props are employed in a gallery, for example, to support a roof cap at a long wall face to be mined in the gallery between the long wall region and the structure behind the mined wall and in drifts or tunnels generally.
By rotation of a threaded spindle, for example, relative to a nut on a post, the prop can be braced between floor and ceiling to withstand the ceiling loads.
Such a prop will act as a rigid post until a predetermined supporting force is overcome. To prevent overloading of the post when the maximum sustainable force is exceeded, the parts of the prop can telescope relative to one another while resisting the movement of the ceiling toward the floor. The prop thus has a certain length over which it can contract controlledly while providing its maximum resistance at the level of the aforementioned maximum force.
Known coal props which can be braced between the floor and roof of the mine gallery are described in the Dywidag brochure published by Dywidag Systems International of Lamont, Ill., and entitled "Dywidag Coal Post, the Yieldable Roof Support System."
In this arrangement, the post is a one-piece tube which receives the spindle nut having a conical conformation.
To brace the prop between the floor and roof, the threaded spindle is rotated. This can be a time-consuming difficult procedure which may not be carried out conveniently. When the maximum load is exceeded, the conical spindle nut is driven into the tube to spread the latter and the resistance to the axial displacement of the nut into the deformable tube defines the load which is sustained during the telescoping collapse of the prop.
Experience has shown that this system does not provide a precisely defined contraction force or resistance to the load which is inducing the collapse of the prop. Furthermore, since the maximum length to which the prop can collapse while providing the aforementioned resistance to such collapse is defined by the length of the spindle, the system requires a very long spindle which can be an expensive part of the device.
Finally, since increasing spindle length means an increased tendency to buckling of the prop along the spindle, the prop has questionable reliability at least when the spindle is fully extended.
Another type of post or prop described as a telescoping support column is described in U.S. Pat. No. 3,737,134. In this system, the post comprises two telescopingly interfitted tubular elements between which a conical member is provided and which generates the resistance to collapse and hence the collapsing force as one tube telescopes into the other when the maximum load on the prop is exceeded. At the top of the upper tube, a spindle is provided which can be rotated to brace the prop against the roof and floor. The spindle may be provided with a cap or a plate which can be attached to a wooden beam or the like.
This system is also not free from the danger of collapse, has the same difficulties with respect to bracing the prop and does not fully resolve all the problems encountered with the earlier prop.